On June 3, 2021, first lady Jill Biden celebrated her 70th birthday. In a tweet marking the occasion, CNN reporter Kate Bennett noted that Biden was the "oldest sitting first lady in modern United States history."
This is true. Biden is the oldest woman to hold the position of first lady.
When U.S. President Joe Biden took office in January 2021, he became the oldest man to hold the position of commander in chief. And by his side, Jill Biden was the oldest woman to become first lady. Biden is the first septuagenarian first lady. Nancy Reagan, Barbara Bush, and Bess Truman all left the White House before their 68th birthdays.
On the other end of the spectrum, Sarah Van Buren was the youngest woman to become first lady at the age of 20 in 1838. Sarah Van Buren, who was President Martin Van Buren's daughter-in-law, took on the duties of first lady as Van Buren's wife had passed away. The youngest woman to be married to a sitting president was Frances Cleveland, who became first lady at the age of 21 in 1886.
While it wasn't uncommon for first ladies to be in their 20s or 30s in the 1800s, the average age for modern first ladies is about 50-years-old. CNN reported:
The average age of a first lady in the White House for the last several decades is 50; Helen Taft, Grace Coolidge, Eleanor Roosevelt, Rosalynn Carter, Hillary Clinton, Michelle Obama and Melania Trump all marked their 50th years during their husbands' administrations.